What is the difference between ‘include’ and ‘prepend’ in Ruby?

Question

What features of Module are defined as append and prepend?

As specified in the text you quoted:

the constants, methods, and module variables

How they differ functionally?

Both add methods of the mixed-in module to the passed module (class). The difference is in the lookup order of these methods, in case that the target class already has them defined:

include behaves as if the target class inherited mixed-in module:

module FooBar
  def say
    puts "2 - Module"
  end
end

class Foo
  include FooBar

  def say
    puts "1 - Implementing Class"
    super
  end
end

Foo.new.say # =>
            # 1 - Implementing Class
            # 2 - Module

prepend makes the methods from the mixed in module “stronger” and executes them first:

module FooBar
  def say
    puts "2 - Module"
    super
  end
end

class Foo
  prepend FooBar

  def say
    puts "1 - Implementing Class"
  end
end

Foo.new.say # =>
            # 2 - Module
            # 1 - Implementing Class

The example kindly ripped off from here: http://blog.crowdint.com/2012/11/05/3-killer-features-that-are-coming-on-ruby-2-0.html

When to use append_features and when prepend_features?

Use prepend when you want to keep methods of the target module (class) at the end of the method lookup chain.

Some real-world examples can be found by searching SO for ruby, module and prepend:

Overriding method by another defined in module
When monkey patching a method, can you call the overridden method from the new implementation?
Ruby: Module, Mixins and Blocks confusing?

(Note: I am mentioning only methods, as they are easiest to picture when it comes to inheritance and mixing-in, but the same applies to other features.)

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